Lithium alloy/iron sulfide bipolar stacked cell batteries have positive and negative electrode materials confined relative to structural positive and negative current collectors which are electrically insulated from one another by separators. The negative electrode material may be a lithium alloy (LiAl); the positive electrode material may be iron sulfide (FeS.sub.1 FeS.sub.2). The separator may be formed of a powder magnesium oxide (MgO). Typically, the electrolyte may be a lithium chloride, lithium bromide, and potassium bromide mixture (LiCl-LiBr-KBr). Current collectors of electrically conductive sheets also function to confine the electrode materials.
Batteries comprised of multiple cells grouped together face to face, and electrically connected in series, are capable of very high current density and have high energy density. Such batteries operate at temperatures ranging from 350.degree. C. to 500.degree. C. At such temperatures, the electrode materials and electrolyte are highly corrosive so that the current collectors must be corrosive resistant while electrically conductive. Such batteries are required to have long operating life in excess of 1000 "deep discharge" cycles. During such cycling, the positive and negative electrode materials undergo volumetric changes, thereby shifting the electrode materials within the battery cell. This can result in deformation of the separators.
Additionally, electrolyte leakage has been experienced between positive and negative electrodes of adjacent cells.
In the past, compression of the stacked and sandwiched cell components within an exterior battery case has been employed to maintain the separator sealed at its peripheral edge. Attempts at hermetic sealing of the bipolar battery have been ineffective.
Attempts have been made to overcome such problems in high temperature lithium alloy/iron sulfide bipolar batteries as exemplified by U.S. Pat. No. 4,687,717 issued Aug. 18, 1987, to Thomas D. Kaun et al. and entitled "Bipolar Battery with Array of Sealed Cells."
In U.S. Pat. No. 4,687,717, the problem has been met by forming initially a plurality of cell and closing assemblies, each comprised of an electrolyte separator sheet, a pair of perforated metal sheets on opposite sides of the separator sheet, with the perforations being within a predetermined perimeter, and an insulating member between and sealed to the metallic sheets and about the perimeter to enclose the separator sheet at the perimeter, providing electrolyte to fill each electrolyte cavity, inserting the electrically opposite electrodes on the perforated metal sheets on sides opposite the separator sheet, assembling the cells and cell enclosures in an array with a current collector sheet between adjacent cells, and an end face cap on each opposite end of the array. The assembling step includes sealing each metal sheet to an adjacent current collector sheet or end face cap to enclose the adjacent electrode. Finally, the array is enclosed within an external housing and electrical connections are provided to electrically opposite ends of the array.
Such approach is characterized by the formation of cup-like electrode holders via the peripheral edges of the sheets which project outwardly beyond the spacer and traverse the side edges of adjacent electrode material and fusing the cuplike electrode holders to the adjacent current collector or end face members of the array.
Such approach to hermetic sealing of the bipolar battery by forming thermal compression seals for each cell is expensive and time consuming. Besides, that approach still requires the stacked array to be placed in an external housing or container which functions to enclose the array and provide electrical connection to the electrically opposite ends of the array.
An attempt has been made to further improve such bipolar battery cell, having thermal compression seals for each cell, with leak proof, sulfide ceramic seals of the positive and negative electrode materials, where the seals adjacent the electrolyte cavity or chamber are formed prior to the addition of electrolyte where the repeating, cup-like design of the battery allows the battery components to be assembled incrementally from the open end of a sealed external container or housing, wherein telescoping seal elements capture the periphery of the separator of each cell to enhance cell durability, where the battery current collector element or collector elements are of diaphragm form and displaceable to accommodate changes in electrode volume during electrical charging and discharging cycles, where refractory metal-coated steel components contain the electrodes, where the bipolar battery structure has a combination of external ceramic rings and sealants at each cell to resist humidity and provide electrical insulation, and where high strength bonds between metals and sulfide ceramics have employed modification of the bond interface to form an intermediate material such as an aluminide, silicide, or phosphide.
While such approaches have been capable of alleviating the leakage of electrolyte at high temperature operation of the bipolar batteries and the maintenance of the structural integrity of the bipolar battery, the requirements of sealing the periphery of cup-like cells of the stacked array, the necessity for individual leak-proof ceramic seals for each cell of the stack, and the necessity of a cell enclosure for the stacked array of cells increases greatly the cost of the bipolar battery, the complexity and structure, and the time and effort in the manufacture of such bipolar batteries.
It is, therefore, an object of the invention to provide an improved bipolar battery which eliminates the need for individual leak-proof sulfide ceramic seals for the individual cells, eliminates the necessity of a repeating cup-like cell configuration and the incremental assembly of the same into a multi-stack array from an open end of a sealed auxiliary support and containment structure, and the elimination of external ceramic rings and sealants to encapsulate the battery to resist humidity and provide electrical insulation.
It is a further object of the invention to accomplish simultaneously the sealing of a plurality of cells of a bipolar battery stack and form a structural container for maintaining the stack under compression which is stable, and capable of absorbing thermal-induced expansion of the cell components during battery operation and cyclic drain and recharging all at high operating temperatures, in a single low-temperature operation, which forms an inter-cell seal preventing the flow of electrolyte from cell to cell at elevated temperatures and over long periods of time, and which locks in cell dimensions.